Jan. 15, 1 8 74 J 



NATURE 



207 



[A tube from an air-pump was then attached to a 

 pipe communicating with the interior of the cylinder, 



quires no time at all to pass through space. This great 

 problem has also been solved ; and we now know that 

 while sound moves at the rate of i,ogoft. a second, light 

 passes over the almost incredible distance of iS6,ooo 

 miles in the same time. Hence at the distances employed 

 in our observations, our watches were entirely unable to 

 inform us that light required any time at all to pass 

 through space. 



But if I stopped here, your next question would be — 

 What is this thing which passes through the air with a 

 velocity of ijOgoft. a second, and wliich, when it reaches 

 us, makes us hear an explosion .' We must give a 

 thorough and complete answer to this question, but to do 

 this wc need a little preparation. Like sailors going into 

 battle, we must clear our decks for action ; and here I 

 must ask you to give me your patient and resolute at- 

 tention. 



In order to know how sound is propagated through the 

 air, we must first know something regarding the air 

 itself. Let us examine the air. 



First, the air has weight. It presses upon a single 

 square foot of this table with the weight of nearly a ton 

 (144 X 15 = 2,160 lbs.). I have here a glass cylinder 

 covered at the top with a sheet of india-rubber. The air 

 presses on that surface with the weight of nearly 900 lbs. 

 But then you will ask how the india-rubber bears it. 

 Why is it not pressed in ? Because air is on both sides 

 of it, and the pressure on the inside is exactly equal to 

 that on the outside. But if I take away the air from the 

 inside of the cylinder, you will soon see the india-rubber which stood on a brass plate, to which its edges were 

 pressed down by the weight of air above it. , ground parallel ; the pump was set in action, and the 



SECTION AT a 

 ri.AME JUMPING 



india-rubber diaphragm at once sank down, in the end 

 clinging to the sides of the glass, forming a deep vessel 

 lining the inside of the cylinder.] 



When the air is let in again, you observe the rubber 

 returns slowly to nearly its primitive position ; it would 

 entirely, but that the india-rubber is a little over-stretched. 



We have thus seen the effect of removing the pressure 

 from the inside. What would occur if we took the out- 

 side pressure away ? The india-rubber would expand. 

 Instead of trying to remove the whole of the air from this 

 room, which is impossible, I will cover these two slack 

 and collapsed bladders with this glass vessel, fitting 

 accurately on to the plate, over which they are suspended ; 

 and then draw off by the air-pump the air surrounding 

 them. See how they gradually blow out ; the folds are 

 now nearly abolished ; now they have become quite 

 smooth. 



Why is this .' Because the air particles have the power 

 of pushing one another apart, and thus take up sufficient 

 space to fill the bladders when the external pressure is 

 removed. The air in this room is pressed upon by the 



weight of the whole atmosphjie. The repelling force 

 which the air particles exert upon each other is called 

 the elastic force of the air. 



Now we have to consider how the sound of the gun is 

 propagated through air. Does the gun fire anything 

 through the air ? No. We may in a rough way represent 

 the particles of air by the solitaire balls arranged in a 

 row close together in this groove. I take the first one 

 and roll it against the second. You observe the row docs 

 not move, only the end one gees away. The first delivers 

 up its motion to the second, and then stops, the second 

 delivers its motion to the third, the third to the fourth, and 

 so on until the last, which, meeting no resistance, flies off. 

 In this way we may figure the motion as transmitted from 

 particle to particle of the air. 



A still better idea may be derived from this model (Fig. 

 i), which has been devised by the ingenuity of my 

 assistant, iVIr. Cottrell. 



In my hand I hold a stem (.A.), passing through the up- 

 right (B), by which a shock can be sent from a ball (C) 

 through a spring to another ball, thence through another 



